1. Field of the Invention
The present invention relates to an LED array head for use with an apparatus such as an LED printer using an electrophotography process.
2. Description of the Prior Art
FIG. 3 shows a schematic explanatory diagram of an LED printer including an LED array head.
In the structure of this diagram, reference numeral 1 indicates a photosensitive drum made of a photoconductive material and rotates in a direction indicated by an arrow mark. The drum 1 is uniformly and electrically charged by means of a charger 2 and is then subjected to an exposure by an LED array head 3 according to the recording image information, thereby forming an electrostatic latent image on the drum 1. In a developer 4, the latent image is developed to attain a toner image, which is transcribed or transferred onto a sheet of paper 6 by an operation of a transcriber or transfer unit 5. The sheet of paper 6 onto which the toner image has been transferred is fed to a fixing apparatus 7, where the toner image is fixed on the sheet of paper 6. On the other hand, after the transfer operation is finished, the photosensitive drum 1 is cleaned by a cleaner 8 for subsequent usage.
The LED array head 3 is, as will be described later, includes an array of LED elements in which a lot of light-emitting diode (LED) chips are arranged in a direction of the character printing operation and a rod lens array of the self-convergence type for converging light radiated from the LED elements of the LED array and for exposing the photosensitive drum 1 to the light.
In the prior art LED array head 3 used in the LED printer above, as shown in FIG. 4, an LED chip 22 is disposed on a ceramic substrate 21 by die bonding and wire bonding is applied to each LED light emitting section so as to supply electric power thereto. Moreover, a cover glass 26 is mounted on a heat dissipating plate 23 made of aluminum or the like to protect bonding wires connected to the LED chip 22 and a conductor pattern on the ceramic substrate in which the LED chip 22 is mounted, and a retaining member 25 retaining the rod lens array of the self-convergence type is supported on the heat dissipating plate 23, thereby configuring the LED array head.
In the LED array head described above, when mounting an LED chip on the ceramic substrate, a positioning precision of about 10 .mu.m is necessary, for example, with respect to the linearity of the positioning. Due to the large dimensions of the chip width and the width of the die bonding pad, for example, the chip width is about 0.7-1 mm and the dibonding pad has a width of about 1.2-1.5 mm, an optical method using a TV camera or the like is indispensable for the positioning operation, which deteriorates the efficiency of the die bonding of the LED chip and further requires use of an expensive apparatus to mount the LED chip.
In addition, the wire bonding must be effected one time for each light emitting section of the LED (usually, 64 light emitting sections are formed in an LED chip). For example, for an LED array of an A4-letter size manufactured with an ordinary integration density of 300 DPI (about 12 dots/mm), 2560 wire bonds are necessary. In this case, consequently, even when a high-speed wire bonder is adopted, a period of time of about 40 minutes to one hour is required for the wire bonding operation. Furthermore, if there exists even one wrong bonding in the 2560 wire bonds, the overall LED array is assumed to be rejected and hence this operation must be effected with a special care, which lowers the production efficiency and causes the production cost to be increased.
Moreover, since a high precision of .+-.100 .mu.m is required for alignment of the LED chip of the LED array head and the fluctuation of the linearity of the light emitting sections in the direction of the height thereof each with respect to the direction of the width (216 mm for the A4-sized LED array head), the warping and bending of the ceramic substrate itself in restricted. As a consequence, a high-precision ceramic substrate is necessary and each ceramic substrate must be subjected to a selection; consequently, the price of the substrate is soared.
In addition, since the rod lens array of the self-convergence type is retained on a heat dissipating plate with a ceramic substrate disposed thereon, there may appear a disadvantage that the dimensional errors of these components are accumulated and hence the necessary precision cannot be obtained. At present, the positioning precision of the rod lens array of the self-convergence type is .+-.0.1 mm along the entire length of the rod lens array, it is indispensable to optically confirm the positioning precision and to provide the LED array head with a mechanism enabling the high-precision positioning, which leads to an increase of the production cost associated with the number of assemblage processes, the manufacturing facility, the number of components, and the like.